U.S. patent application Ser. No. 15/638,052 filed 29 Jun. 2017 entitled “Antenna Loaded with Electromechanical Resonators” relates to the use of arrays of electromechanical resonators in connection with antennas. Some the embodiments disclosed therein may include applications in higher frequency bands (e.g., 3 to 10 MHz) for small handheld portable wireless systems. In addition to increasing the bandwidth of such systems, frequency hopping using FSK (Frequency Shift Keying) is also disclosed. This may use multiple HF resonators that can be operated at moderate power levels (˜1 W) each with a different mechanical resonant frequency. A new HF MEMS-based resonator design and manufacturing method is disclosed which allows the resonators to be fabricated in an array on a single thermally conductive substrate to minimize heating and allow wafer-level packaging to reduce the size of the inductor array. Moreover, the design and manufacturing allows integration with switches and other electronics needed for antenna matching and frequency shifting.
Previous VHF inductors used for antenna matching have consisted primarily of wire coils (several mm3 in volume) with Quality Factors (Qs)<50 and inductances in the 100's of nH range. As the frequency decreases, the matching inductances for antennas increases and the resistance tends to increase unless the coil becomes very much larger (>5000 cm3). In the HF frequency range, traditional compact commercial coils have Qs<50, inductances of 0.1 to 1 mH, volumes of 100 mm3 and weight of 0.5 gm. The quartz-based inductors disclosed herein can have Qs=600, inductances of 1 to 10 mH, volumes of about 4 mm3 and a weight of about 5×10−3 g. In addition, using the disclosed MEMS-based process, arrays of inductors can be fabricated on the same chip with different resonant frequencies for improving the bandwidth. The substrate can be chosen to incorporate electronics for frequency switching or antenna matching and/or can be chosen for high thermal conductivity for minimized heating during high power operation. Finally, the disclosed quartz inductors will be fabricated from temperature compensated cuts of quartz, thereby providing high frequency stability of 10s of ppm over typical temperature ranges of −20° to +100° C. This is several orders of magnitude better than that obtained with other MEMS resonators such as Si or AlN.
Some of the embodiments disclosed herein may be useful in LF and VLF antenna systems for GPS-denied applications, such as, underwater communication, and long-range ground communication systems. At higher frequencies, HF communication links are of great interest for ground deployed military forces due to their ability to travel long distances, penetrate more easily through cluttered environments, and relative immunity to compact radio-monitoring techniques. Thus, high-Q electromechanical devices with high inductances may play an important role in next generation transmitter systems for a variety of frequency ranges and systems. In particular, the disclosed quartz MEMS high-Q inductors may provide enabling capabilities for RF transmitters in a variety of frequency ranges.
In addition, HF oscillators with low phase noise can be constructed from the disclosed resonators and used for timing references. Due to their lower resonant frequency than previous shear-mode resonators made by HRL Laboratories LLC, the Q of the disclosed resonators are expected to be higher. This results in lower phase noise and Allan Deviation. Precision navigation and timing applications consistently demand higher levels of frequency stability.
Extensional-mode quartz resonators have been manufactured by several quartz manufacturers in the frequency range of 500 kHz to 32 MHz. However, in order to produce an electric field in the crystalline x direction for exciting the extensional mode, the previous designs used electrodes which are wrapped around the edges of the quartz plate, similar to a tuning fork design. The shadow masking technique using for the previous devices is hard to implement in a wafer-level process and can lead to misalignments and low yield. Our new design makes use of opposing edge electrodes described in U.S. patent application Ser. No. 14/973,701, identified above, with vias in the quartz to connect the top electrodes to a bottom-side bonding pad. A new bonding pad geometry is described in which the pads are positioned on tethers and bonded to the substrate in close proximity to each other on opposite sides of the narrow portion of the resonator for each signal polarity. This reduces the stress between the quartz and the substrate, and the parallel connections for each electrode of similar polarity reduce the interconnect resistance, which helps in maintaining high Q. By attaching the bonding pads to tethers and placing the bonding pads in close proximity, the bonding yield is improved and arbitrary high thermal conductivity substrates can be utilized for removing heat from the resonator. Folded springs are proposed for attachment between the resonator and the tethers for minimizing modal energy losses to the substrate and maintaining high Q.
Previous extensional-mode resonators have been fabricated with wet-etching techniques using small quartz blanks thus producing highly asymmetric sidewall profiles around the resonator plate. The proposed process is compatible with HRL quartz-MEMS wafer-level processes (see U.S. Pat. No. 8,765,615) in which dry plasma etching is used to define the quartz pattern in roughly 100-μm-thick quartz bonded to a handle wafer and the improvements suggested by U.S. Provisional Patent Application No. 62/522,573, which produce near vertical profiles for tighter dimensional control on the design and a better performance match to simulation. This wafer-level process, in turn, provides for wafer-level encapsulation, which is not possible for commercial quartz resonators. This leads to a smaller and lighter overall package. U.S. Pat. No. 8,765,615 and U.S. Provisional Patent Application No. 62/522,573 are hereby incorporated herein by reference.
For extensional mode devices, the dimension of the resonator along its lateral x-axis determines its frequency. We vary the width of the resonators across a wafer to produce a range of resonant frequencies for frequency switching applications in a small, low-cost package.
The use of quartz resonator for high-Q inductors for distributed loading antennae to improve their efficiently for low frequency RF transmitters has recently been of interest. See the discussion of U.S. patent application Ser. No. 15/638,052 above. However, the power handling of small quartz resonators can be an issue. Typically, VHF shear-mode quartz resonators become nonlinear and their phase noise properties degrade for oscillator applications when voltages exceed roughly a volt across the resonator at powers of ˜1 mW. In addition, heating can shift the series resonance by 10 s of ppm. For critical timing applications this can be problematic. However, using extensional mode resonators at lower frequencies, the distance between driving electrodes can be increased by as much as a factor of ten compared to common VHF shear-mode resonators (e.g., 600 μm compared to 60 μm). Moreover, for antenna coupling applications, a degradation in the linearity and small shifts in the series resonance can often be tolerated. In addition, by using designs which allow integration on highly thermally conductive substrates, such as thin copper sheets, one can design extensional-mode resonators with high Q, low temperature sensitivity, and small size that can withstand a higher power (˜1 W) than has previously been assumed.
These resonators, at lower power levels, can also be used for arrays of HF oscillators on a single chip if integrated with typical sustaining circuits in the substrate using the HRL quartz MEMS processes mentioned above.